Wild conspiracy theories about Malaysian Airlines flight MH370 have now spread to the search ship tasked with finding it.

For nearly a week some aviation buffs and MH370 followers have been debating online whether the missing plane has in fact been secretly found and — if not — why the ship's Automatic Identification System (AIS) was abruptly turned off for several days, preventing online observers from tracking its movements.

The ship, Seabed Constructor, suddenly went "dark" on tracking websites not long after it had completed a curious circle, several kilometres wide, prompting many on Twitter to question what was inside the circle on the sea floor.

The ship then headed south-west in a straight line, and a few kilometres later turned its AIS off.

"I'm sticking with my theory that the big circle is a piece of debris, and the line south was to locate the plane. When they think they found it they turned off AIS as protocol," one tweet said.

"This. Is. Strange. I have never seen a ship do this. Maybe there's an AUV lost down there?!?" said another.

Seabed Constructor has spent two weeks scouring the ocean floor in the southern Indian Ocean for the fuselage or debris from MH370.

Its operator Ocean Infinity — a Texas-based company — has signed a deal with the Malaysian Government to search a 25,000-square-kilometre area over 90 days, and will receive payment of between $US20 million and $US70 million only if it finds the missing plane.

Speculations ship made secret detour to chest

The decision to switch off the AIS prompted some to speculate that the ship had made a secret detour to a nearby shipwreck to retrieve a chest known to be on the sea floor.

The shipwreck was discovered in 2015 during the previous Australian-led search for MH370, in waters south-west of the current search zone.

The ship itself has all but dissolved over time, leaving only the metal frame and piles of nuts and bolts.

But Paul Kennedy, chief executive of Fugro — the company that carried out the first undersea search — confirmed in 2016 that a large chest was the only thing left intact.

"It's a big chest, it's about three metres long, maybe one-and-a-half metres wide. And it's still closed," he told a conference in Perth.

"The whole ship has deteriorated. But there's a big chest in about 4,000 metres of water."

The ship's identity has not been confirmed, so it is impossible to know what, if anything, is in the chest.

Aviation buff, John Zwicker, tweeted the chest "may be a box of old socks".

Mr Kennedy said the WA Maritime Museum had no records of a ship that matched the wreck found.

But the anchor had "ceased manufacture" about 1820, meaning the vessel could be almost 200 years old.

Others have speculated that it could be a Peruvian-built transport ship, the S.V. Inca, which disappeared on its way to Australia in 1911.

Either way, Twitter has run hot with speculation that Ocean Infinity indeed took a deliberate detour to the wreck, presumably to retrieve the chest and any booty it might contain.

"Tomorrow I'll confirm the GPS I have for #Constructor down to the wreck and back. I've already confirmed it with my source. It happened. It isn't a big deal from my point of view," said Mike Chillit, a long-time MH370 follower.

He questioned whether Australians had a right to share the spoils of any bounty brought up from the deep.

Others are sceptical, given the strict 90-day deadline Ocean Infinity has to find MH370 if it wants to receive any payment.

"I don't see the point of OI going to have a look at the shipwreck now. They have a 90-day window, Malaysian 'observers' on board and a target: #MH370. They can look at it after the search if interested," aviation buff Juan Valcarcel said.

Seabed Constructor to dock in WA

Ocean Infinity has repeatedly declined media requests for interview, so it may never reveal why it turned its AIS system off, or whether it used the three to four "silent" days to visit the shipwreck.

The AIS was turned on only after the Seabed Constructor was apparently on its way to Fremantle in WA. It is due in port within the next 48 hours.

A spokesman for the company has told the ABC that the stop is "a quick turnaround of the vessel and then continuing with the search".

Aviation experts say even with the AIS turned off, the ship is still visible on marine radar systems, but not on live website tracking apps.

The Malaysian Government has not commented on the search for MH370 since January 30, when it reported only that "no significant contacts" had been identified to date.